Substrate-controlled allotropic phases and growth
orientation of TiO2 epitaxial thin films
V. F. Silva, Valérie Bouquet, Stéphanie Députier, S. Boursicot, Sophie
Ollivier, I.T. Weber, V. L. Silva, I. M. G. Santos, Maryline Guilloux-Viry,
André Perrin
To cite this version:
V. F. Silva, Valérie Bouquet, Stéphanie Députier, S. Boursicot, Sophie Ollivier, et al..
Substrate-controlled allotropic phases and growth orientation of TiO2 epitaxial thin films. Journal of Applied Crystallography, International Union of Crystallography, 2010, 43, pp.1502-1512.
<10.1107/S0021889810041221>. <hal-00825767>
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Journal of
Applied
Crystallography
Substrate-controlled allotropic phases and growth
orientation of TiO2 epitaxial thin films
ISSN 0021-8898
Received 21 May 2010
Accepted 13 October 2010
V. F. Silva,a,b V. Bouquet,a* S. Députier,a S. Boursicot,a S. Ollivier,a I. T. Weber,c
V. L. Silva,b I. M. G. Santos,d M. Guilloux-Virya and A. Perrina
a
Sciences Chimiques de Rennes, UMR 6226 CNRS/Université de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France, bLEAQ, Departamento de Engenharia Quimica, Universidade Federal
de Pernambuco, CEP, Brazil, cDepartamento de Quı́mica Fundamental, CCEN, Universidade
Federal de Pernambuco, CEP, 50740-540 Recife, PE, Brazil, and dLACOM, Departamento de
Quı́mica, Universidade Federal de Paraiba, Campus I, CEP, 58059-900 João Pessoa, PB, Brazil.
Correspondence e-mail: [email protected]
# 2010 International Union of Crystallography
Printed in Singapore – all rights reserved
TiO2 thin films were grown by pulsed laser deposition on a wide variety of oxide
single-crystal substrates and characterized in detail by four-circle X-ray
diffraction. Films grown at 873 K on (100)-oriented SrTiO3 and LaAlO3 were
(001)-oriented anatase, while on (100) MgO they were (100)-oriented. On (110)
SrTiO3 and MgO, (102) anatase was observed. On M-plane and R-plane
sapphire, (001)- and (101)-oriented rutile films were obtained, respectively. On
C-plane sapphire, the coexistence of (001) anatase, (112) anatase and (100)
rutile was found; increasing the deposition temperature tended to increase the
rutile proportion. Similarly, films grown at 973 K on (100) and (110) MgO
showed the emergence, besides anatase, of (110) rutile. All these films were
epitaxically grown, as shown by ’ scans and/or pole figures, and the various
observed orientations were explained on the basis of misfit considerations and
interface arrangement.
1. Introduction
Titanium dioxide has attracted considerable attention because
of its special properties, such as large energy gap, high
refractive index and high dielectric constant. This oxide exists
in three main crystallographic phases: rutile (tetragonal),
anatase (tetragonal) and brookite (orthorhombic) (Banfield et
al., 1993). Anatase is known to be the low-temperature form of
TiO2 and it transforms irreversibly to the most stable rutile
phase upon heating (Shannon & Pask, 1965), whereas brookite is rarer in nature and difficult to synthesize. TiO2 is
commonly used in optical devices, corrosion protective and
self-cleaning coatings, sensors, and catalysts (Diedold, 2003;
Walczak et al., 2009). A better understanding of catalytic and
photocatalytic processes is one of the main driving forces for
TiO2 surface investigation (Diedold, 2003).
To grow high-quality TiO2 films various deposition methods
have been used, such as atomic layer deposition (Aarik et al.,
2001; Heikkilä et al., 2009; King et al., 2008), chemical vapor
deposition (Sobczyk-Guzenda et al., 2009; Kim et al., 2010; Sun
et al., 2008), sputtering (Singh et al., 2008; Meng et al., 2009;
Heo et al., 2005), electron beam evaporation (Lotnyk et al.,
2007) and pulsed laser deposition (PLD; Zhao & Lian, 2007;
Walczak et al., 2009; Yamamoto et al., 2001). Crossing the
published data, we can deduce that all these methods allowed
the growth of epitaxial TiO2 thin films on various substrates
provided that a careful optimization of the deposition condi-
1502
doi:10.1107/S0021889810041221
tions was carried out. However, owing to the variety of the
reported experimental conditions, it appears quite difficult to
extract the specific contribution of the nature and orientation
of the substrates.
The present investigation focuses then on the effects of
substrate on epitaxial growth of TiO2 thin films prepared by
just one method, PLD, on a large selection of single-crystal
substrates. The objective is to achieve accurate control of the
allotropic variety and orientation of TiO2 thin films, which will
open the way to a comparative study of the surface effect on
their behavior. In fact, it is expected that surface characteristics, morphology and crystallite orientations, inducing the
development of active facets, would be of primary importance
for applications such as photocatalysis and gas sensors
(Lotnyk et al., 2007). It is believed that knowledge of the
surface properties on a fundamental level would help to
improve material properties and device performance
(Diedold, 2003). In particular, high-quality epitaxial anatase
and rutile surfaces can be quite useful for understanding the
photocatalytic mechanisms, independently of defect contributions (Yamamoto et al., 2002).
2. Experimental
TiO2 thin films were deposited by PLD on various substrates.
Deposition was carried out using a KrF excimer laser (Tuilaser
Excistar, pulse duration of 20 ns, = 248 nm). The laser beam,
J. Appl. Cryst. (2010). 43, 1502–1512
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incident at 45 , was focused in a standard vacuum chamber
(background pressure of about 0.5 mPa) on a rotating target,
whose surface was mechanically polished before each
deposition run. The laser fluence was 2 J cm2 with 2 Hz
repetition rate. The distance between the target and the
substrate was 42 mm. The deposition temperature was usually
fixed at 873 K, but other temperatures (773–973 K) were also
tested in specific examples. The oxygen pressure was 10 Pa and
the deposition time was typically 30 min, leading to films about
100 nm thick.
A homemade target of TiO2 rutile (99.8% purity) of 2.3 cm
diameter was used, which was fabricated by sintering the coldpressed TiO2 powder pellet at 1373 K for 10 h in air. Single
crystals of (100)-oriented SrTiO3 (STO), MgO and LaAlO3
(LAO), as well as (110)-oriented STO and MgO, and R-, Cand M-sapphire were used as substrates. Just before deposition, substrates were sonically cleaned in acetone and
2-propanol and air dried.
After deposition, samples were cooled under an oxygen
pressure of about 3 kPa. No further ex situ annealing was used.
All films appeared highly glossy and fully transparent, without
cracks or macroscopic defects.
Standard –2 X-ray diffraction (XRD) scans were
performed with a two-circle Bruker D8 diffractometer using
monochromated Cu K1 radiation. Rocking curves, ’ scans
and asymmetric diffraction data were recorded with a fourcircle Bruker D8 Advance diffractometer equipped with an
Euler cradle and a Göbel mirror selecting a parallel beam of
Cu K radiation.
The surface morphology of the thin films was observed with
a Jeol 6301-F field emission scanning electron microscope
operated at low voltage (typically 9 kV) in order to limit
charge effects and to achieve a high resolution without the
need of surface metallization.
3. Results
In the following X-ray diffraction patterns, indexations refer
to the tetragonal anatase phase [a = 3.7852, c = 9.5139 Å, space
group I41/amd (141), according to JCPDS 21-1272] or rutile
phase [a = 4.5933, c = 2.9592 Å, space group P42/mnm (136),
according to JCPDS 21-1276].
3.1. TiO2 film growth on (100)-oriented cubic substrates
Figure 1
–2 XRD patterns of TiO2 thin films deposited at 873 K on (100)oriented substrates: (a) STO, (b) LAO and (c) MgO. Diffraction peaks
labeled A refer to the anatase phase. Peaks marked with a star (?) or a
hash sign (#) are related to substrates and the sample holder, respectively.
Note the logarithmic intensity scale.
J. Appl. Cryst. (2010). 43, 1502–1512
Fig. 1 displays the –2 XRD patterns of the films deposited
at 873 K on various substrates. Figs. 1(a) and 1(b) refer to films
grown on (100)STO and (100)LAO, respectively. They reveal
only peaks due to (004) and (008) anatase, whereas Fig. 1(c)
displays the (200) and (400) anatase diffraction peaks on
(100)MgO. In summary, for the (100)-oriented substrates we
obtained the anatase allotrope of TiO2 alone. In order to
evaluate the crystalline quality and in-plane orientation of
these films, rocking curve and ’-scan analyses were
performed.
Films deposited on the (100)STO substrate are oriented
well out-of-plane (FWHM of the rocking curve ! = 0.84 for
the 004 reflection) and epitaxied as shown by the ’ scan
displayed in Fig. 2(a). The four 90 -spaced peaks relative to
the 103 reflection denote the fourfold symmetry of the (001)V. F. Silva et al.
Substrate-controlled allotropic phases
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oriented film and arise at the same azimuth as the 110 peaks of
the substrate. Then the epitaxial relations are the following:
ð001Þanatase ==ð100ÞSTO and h100ianatase ==h001iSTO :
ð1Þ
The FWHM ’ = 1.3 reflects a very good in-plane ordering
in relation with a misfit of 3%. Even better results are
obtained on LAO substrates, as ! = 0.24 and ’ = 0.39
(not shown), thanks to a misfit of only 0.01%. The epitaxial
relations are the same as above.
Films grown on (100)MgO are, in contrast, (100)-oriented
while exhibiting also well defined out-of-plane ordering (! =
1.54 ). The ’ scan (Fig. 2b) displays four peaks denoting some
in-plane ordering, but their width is as large as ’ = 18 ,
meaning strong in-plane dispersion. The latter could be
related both to a large misfit {10 and +13% for a and c
[referring to (c/2)anatase] anatase unit-cell constants, respectively} and to the competition between two in-plane orientations, defined by [010]anatase==[001]MgO and [001]anatase==
[001]MgO , respectively. The coexistence of these two orienta-
tions is reflected by the observation of four peaks, while the
(100) plane has only twofold symmetry. This coexistence
agrees with a previous report by Hesse & Bethge (1981) in the
case of samples grown between 773 and 973 K by electron
beam evaporation.
Aiming to eventually improve the in-plane ordering, a
sample was deposited at higher temperature, namely 973 K, on
(100)MgO. This resulted in the growth, besides the previous
(100)-oriented anatase, of (110) rutile crystallites (not shown).
From ’-scan analysis it appeared that the anatase in-plane
ordering was not improved (’ = 20 ), while the rutile crystallites were epitaxied (’ = 4 ) with the following in-plane
relationships:
½001rutile ==½011MgO and then ½110rutile ==½011MgO ;
ð2Þ
meaning that the (110) rutile plane is rotated 45 with respect
to the MgO basal square, and the misfits are then 0.4 and 9%
along the rutile [001] and [110] vectors, respectively.
In contrast, decreasing the deposition temperature down to
773 K resulted in the growth of pure (100)-oriented anatase
with a slightly better degree of in-plane ordering (’ = 12 ,
compared to ’ = 18 obtained at 873 K).
Fig. 3 displays field emission scanning electron microscopy
(FE-SEM) secondary electron images for TiO2 films grown on
(100)-oriented substrates at 873 K. The samples present
homogeneous and crack-free surfaces. However, the surface
morphologies are influenced by the nature of the substrate.
The films deposited on (100)MgO, which are not well ordered
according to XRD, appear quite rough (Fig. 3c), while on
(100)STO the TiO2 film appears very smooth despite some
accidental outgrowths (Fig. 3a). The film deposited on
(100)LAO clearly shows well organized square-shaped crystallites (Fig. 3b), the edges of which are aligned with the unitcell vectors of the substrate surface; these crystallites appear
to be self-organized along the striations related to twin
boundaries of the substrate, suggesting that such defects play a
role in the nucleation stage.
3.2. TiO2 film growth on (110)-oriented cubic substrates
Figure 2
’-scan XRD patterns of TiO2 thin films deposited at 873 K on (100)oriented substrates: (a) STO and (b) MgO. The probed reflection is
{101}anatase in the two examples.
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Substrate-controlled allotropic phases
Films were deposited on (110)STO substrates at 873 K and
on (110)MgO at 773, 873 and 973 K. The –2 XRD pattern of
the film obtained at 873 K on (110)STO (Fig. 4a) shows only
one peak at 2 = 62.72 (! = 0.96 ), which can correspond
either to (002) rutile, with a displacement of 2 = 0.02 , or to
(204) anatase, with a displacement of 2 = 0.03 with respect to
JCPDS reference files. In order to resolve this uncertainty,
oblique planes with respect to the basal one were probed.
Indeed, in the first hypothesis the (101) plane of rutile would
diffract at about 2 = 36.08 and (the angle between the
basal plane and the probed one) = 32.82 , while in the second
case the 004 peak of anatase is expected at 2 = 37.80 and =
51.49 . No diffraction was found in the first configuration,
whereas a strong peak appeared for the second one, clearly
evidencing that this film is crystallized in the anatase allotrope.
The full ’-scan pattern taken for the (004) reflection (Fig. 4b)
shows that the film is epitaxied, with two 180 -spaced peaks
J. Appl. Cryst. (2010). 43, 1502–1512
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aligned with the (200) peaks of the substrate, evidencing the
following relationships:
ð102Þanatase ==ð110ÞSTO with h100ianatase ==½001STO
and ½201anatase ==½110STO :
ð3Þ
Because the (102) plane of anatase has no true twofold
symmetry, only one peak would have been expected. The
second one arises from crystallites having their (102) plane,
strictly identical to the previous (102) plane, parallel to the
(110) substrate surface (this twinning mechanism can also be
understood as a 180 in-plane rotation of the crystallites,
which is strictly equivalent): then the two peaks are 180 spaced. The ’ value of 2.3 reflects good in-plane orientation; indeed the misfit between the substrate surface and the
film interfacial plane is about 3% along the [010] direction
and 10% along the [201] direction of the film.
The –2 XRD pattern of the film obtained at 873 K on
(110)MgO did not exhibit any peak in addition to the
substrate peaks (not shown), making it impossible to obtain
any information about the crystallinity and orientation. Then,
complete pole figures were recorded for the (110) rutile and
(101) anatase diffraction planes. No peak was observed in the
first case, while eight discrete peaks appeared in the second
case (Fig. 5), demonstrating epitaxically grown anatase. A
model built from the CaRine Crystallography software
(Bourdias & Monceau, 1998) clearly shows that the film is
(102)-oriented anatase. Four peaks are directly explained by
the model, while the other four are related to a twinning
Figure 3
Figure 4
FE-SEM images of TiO2 thin films deposited at 873 K on (100)-oriented
substrates: (a) STO, (b) LaO and (c) MgO. The arrow on the image
relative to LAO materializes the twin boundaries of the substrate. Inset is
an image of the same film at higher magnification (100 000)
XRD patterns of the film deposited at 873 K on (110)STO: (a) –2 XRD
pattern [peaks marked with a star (?) or a hash sign (#) are related to
substrates and the sample holder, respectively; note the logarithmic
intensity scale] and (b) ’ scan obtained for the probed plane (004)anatase.
J. Appl. Cryst. (2010). 43, 1502–1512
V. F. Silva et al.
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mechanism, as mentioned above. The epitaxial relations are
the same as above on (110)STO,
ð102Þanatase ==ð110ÞMgO with h100ianatase ==½001MgO
and ½201anatase ==½110MgO ;
ð4Þ
with misfit values of 10 and 2%, instead of 3 and 10% for
(110)STO, along the anatase [010] and [201] directions,
respectively. No diffraction was observed on a standard –2
scan because the anatase 102 reflection is forbidden and the
204 reflection is fully masked by the 220 substrate reflection.
The –2 XRD pattern of the film obtained at 973 K on
(110)MgO (Fig. 6) shows two peaks located at 2 = 27.43 and
56.52 , unambiguously assigned to (110) and (220) rutile. The
rocking curve gives large values of ! (6.88 ), meaning very
important out-of-plane dispersion. However, the ’ scan
obtained for the (200) reflection has clearly shown that the
film was epitaxically grown, with quite good in-plane orientation (’ = 2.6 ), a surprising result if one refers to the large
mosaicity of this film. This ’ scan exhibited, as expected, two
peaks, 180 -spaced, reflecting the twofold symmetry of both
the substrate and the film. As these peaks are 90 -shifted with
respect to the azimuth of the substrate {200} reflections, the
epitaxial relationships are
ð110Þrutile ==ð110ÞMgO with ½001rutile ==½110MgO
and then ½110rutile ==½001MgO :
rutile orientations was roughly estimated to be about 0.9, from
the relative intensities of the 004 anatase and the 200 rutile
peaks, integrated over ! and ’ and corrected for powder
relative intensities normalized to corundum and multiplicities.
It is worth noting that some samples deposited at 973 K
exhibited a very weak and broad peak at 29.95 , in contrast to
films grown at 873 K. This peak could be attributed to either
the 121 reflection of brookite or the 220 reflection of Mg2TiO4
(cubic, a = 8.4409 Å). From ’-scan experiments, no oblique
plane of brookite was observed, while the 311 and 111
reflections of Mg2TiO4 were clearly detected. This interfacial
phase is then also epitaxically grown, and grows cube-on-cube
on MgO. The misfit with respect to the substrate is only 0.2%;
then the misfit between TiO2 and MgO or Mg2TiO4 is essentially the same. This result fully confirms the observation of a
topotactic growth of Mg2TiO4 on MgO evidenced by Hesse &
Bethge (1981) at temperatures above 1073 K, on the basis of
transmission electron microscopy.
For a lower deposition temperature (773 K), the film
appeared to be (110)-oriented anatase, with a quite large outof-plane distribution (! = 5 ) but standard in-plane ordering
(’ = 3.5 ). From ’ scans recorded for the 200 reflection, the
epitaxial relations are
ð110Þanatase ==ð110ÞMgO with ½001anatase ==½001MgO
ð5Þ
and ½110anatase ==½110MgO :
ð6Þ
In these conditions, 2crutile fits closely the [110] vector of MgO
(misfit = 0.5%) and, in the other direction, 2 [110]rutile
compares very well with 3 [001]MgO (misfit = 3%). Then this
orientation could be stabilized, at least to some extent, by a
near coincidence site lattice (NCSL) mechanism (Kwang et al.,
1980).
An additional set of ’ scans, relative to the 004 anatase
peak, revealed again the presence of the (102) anatase
orientation, undetectable from –2 scans, as previously
mentioned. The volume ratio of the (102) anatase to the (110)
The misfit values are 13% (for two MgO unit cells fitting one
anatase unit cell along their c axes) and 5%, respectively.
However, the film contains again some epitaxial (102)oriented anatase crystallites, as evidenced by the ’ scan taken
around the 004 reflection of anatase. These (102)-oriented
crystallites represent roughly 30% of the total volume as
evaluated from the intensities of the 004 and 200 anatase
peaks, integrated over ! and ’ and corrected for multiplicities.
Figure 5
Figure 6
Pole figure obtained for the 101 peaks of the (204)-oriented anatase film
deposited at 873 K on (110)MgO; one variant of the twin is fully indexed
(A1); the indexations of the second variant (A2), not shown for clarity,
are deduced with respect to the horizontal symmetry plane.
–2 XRD pattern of the film deposited at 973 K on (110)MgO.
Diffraction peaks labeled R refer to the rutile phase. Peaks marked with
a star (?) or a hash sign (#) are related to substrates and the sample
holder, respectively. Note the logarithmic intensity scale.
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To summarize, in the case of these different (110)-oriented
substrates, the growth of (102) anatase on (110)STO corresponds to reasonable misfits, while the hypothetical growth of
(110) rutile would give very important misfits (+7 and 17%
along the two in-plane directions), a reason to stabilize the
former one. On (110)MgO, the (110) rutile presents a very
good fit (0.5 and 3%), while the alternative (102) anatase
growth leads to values of 2 and 10%, favoring strongly the
rutile allotrope when the thermal energy is enough to induce
the phase transition.
Fig. 7 displays SEM secondary electron images for TiO2
films on (110)STO and (110)MgO substrates. Both films
present a very homogeneous and smooth surface with very
small crystallites. There is no remarkable difference related to
film orientation.
of the film grown on (110)STO. In the ’-scan mode, a strong
peak was observed near 2 = 36.08 and = 32.82 ; thus the
film is clearly (001) rutile (refer to x3.2). The full ’-scan
3.3. TiO2 film growth on sapphire single-crystal substrates
Films were deposited on M-plane (1010), C-plane (0001)
and R-plane (1012) sapphire substrates, in most cases at 873
and 973 K. For all substrates, increasing the deposition
temperature results in an increase of crystallinity. Fig. 8 shows
as an example the –2 XRD patterns of films grown at 973 K.
3.3.1. TiO2 films on M-plane sapphire. The –2 scan
(Fig. 8a) shows only one peak (2 = 62.96 , ! = 0.82 ), which
could be attributed either to the 002 reflection of the rutile
variant or the 204 reflection of the anatase one, as in the case
Figure 8
Figure 7
FE-SEM images of TiO2 thin films deposited on (110)-oriented
substrates: (a) STO at 873 K and (b) MgO at 973 K.
J. Appl. Cryst. (2010). 43, 1502–1512
–2 XRD patterns of the thin films deposited at 973 K on (a) M- (b) Cand (c) R-sapphire substrates. Diffraction peak labels A and R denote the
anatase and rutile phases, respectively. The peaks marked with a star (?)
or a hash sign (#) correspond to substrates and the sample holder,
respectively. Note the logarithmic intensity scale.
V. F. Silva et al.
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pattern (not shown) revealed that the film was epitaxied, with
four 90 -spaced peaks reflecting the fourfold symmetry of
c-axis-oriented rutile. One of these peaks appears at the same
azimuth as the 0112 peak of sapphire, leading to the epitaxial
relationships
were observed for some films grown at 873 K. They reflect the
presence of a very small amount, in this case, of in-planeoriented crystallites with [111]anatase==[110]sapphire.
The ’ scan recorded for the (110) plane of the 100-oriented
rutile is displayed in Fig. 9(c). Again six peaks are observed,
ð001Þrutile ==ð1010Þsapphire with h100irutile ==½100sapphire
and ½001sapphire :
ð7Þ
The misfit between arutile and asapphire is = 3.5%, while
that between arutile and csapphire (referring to one-third of the
sapphire c axis) is = + 6%, quite reasonable values that agree
with the observation of epitaxial growth.
Note that the adjusted values of 2 (36.15 , ! = 1.33 ) and
(32.38 ) associated with the 101 reflection of rutile are close
to the expected ones, suggesting that the unit cell is not
significantly distorted. In order to confirm this assumption, –
2 scans were performed in asymmetric mode. The {202}
planes were chosen on the basis of geometric constraints. The
(!1 !2) difference leads to a c/a ratio of 0.636, and, as crutile =
2.952 Å (bulk value: 2.959 Å) from the –2 scan, then arutile =
4.585 Å (bulk value: 4.593 Å). The two unit-cell constants of
the film are both slightly lower than those of the bulk (by
about 0.2%), ruling out a strain effect.
3.3.2. TiO2 films on C-plane sapphire. The –2 scan of the
film grown at 973 K (Fig. 8b) shows, in addition to the
substrate 0006 reflection, two sets of peaks attributed unambiguously to 004 anatase, 112 anatase (weak), 200 rutile and
their first harmonic. All these crystallites are well aligned
along the growth direction, as shown by the FWHMs of their
rocking curve (! = 1.74, 1.23 and 0.11 , respectively).
The crystallites are also in-plane aligned, as shown by the
’-scan patterns (Fig. 9). The ’ scan recorded for the (101)
plane of the 001-oriented anatase is displayed in Fig. 9(a). It
exhibits 12 well defined peaks (’ = 2.3 ), as a result of the
combination of the fourfold axis of anatase and the threefold
axis of C-sapphire. The azimuthal shift of these peaks with
respect to the 0112 peaks of the substrate (marked by arrows
in Fig. 9c) implies that the diagonal of the (001) plane of
anatase is aligned with the a axis of sapphire, leading to the
epitaxial relations
ð001ÞAnatase ==ð0001Þsapphire and h110iAnatase ==h100isapphire : ð8Þ
The corresponding misfit is = + 12%.
The ’ scan recorded for the (101) plane of the 112-oriented
anatase is displayed in Fig. 9(b). It shows six peaks (’ = 3.3 ),
60 -spaced. Three of them appear at the same azimuth as the
0112 peaks of the substrate, implying that the [111] direction
of anatase is aligned with the a and b axes of the substrate. The
three other peaks, shifted by 180 , are deduced by symmetry:
ð112Þanatase ==ð0001Þsapphire and ½111anatase ==h100isapphire :
ð9Þ
The interface plane (112) has a rectangular mesh defined by
the directions [110] and [111] with the corresponding
constants 5.353 and 5.458 Å. The misfit with sapphire is even
worse ( = + 15%) than in the previous case. Note that six very
weak peaks, shifted by 30 with respect to the principal ones,
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V. F. Silva et al.
Substrate-controlled allotropic phases
Figure 9
’ scan of a TiO2 thin film on C-sapphire deposited at 973 K for (a) the
(101) plane of the 001-oriented anatase, (b) the (101) plane of the 112oriented anatase and (c) the (110) plane of the 100-oriented rutile.
Arrows mark the azimuthal positions of the 0112 substrate reflections.
J. Appl. Cryst. (2010). 43, 1502–1512
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from symmetry considerations, but they are significantly
broader than in the previous case (’ = 6 ). Their azimuth
with respect to substrate peaks shows that the c axis of the
rutile crystallites lies at 30 from the a axis of the substrate;
then
ð100Þrutile ==ð0001Þsapphire and ½001rutile ==½120sapphire ;
i:e: ½010rutile ==½100sapphire :
ð10Þ
The misfit is small along the b axis of rutile, = 3.5%, but as
high as 28% along the crutile axis; however, it is worth noting
that three times the substrate mesh fits two times the rutile c
axis within 7%. Then this orientation could be stabilized, at
least to some extent, by an NCSL mechanism (Kwang et al.,
1980).
Note that the ’ broadening is in agreement with the model
proposed by Sbai et al. (2007), based on the competition of the
above in-plane relation and an alternative very close one, for
which [001]rutile==[110]sapphire, deviating from the first orientation by 2.3 .
The influence of deposition temperature was studied in the
773–973 K range. The results are essentially the same for all
samples, with some decrease of the mosaicity when deposition
temperature increased. Some change of the distribution of the
004 anatase, 112 anatase and 200 rutile relative volumes was
also observed, as shown on Fig. 10. As expected, the rutile
volumic fraction increases sharply at 973 K at the expense of
112 anatase (but not 004 anatase, which appears to be more
efficiently stabilized).
3.3.3. TiO2 films on R-plane sapphire. Apart from the
substrate peaks, the –2 scan (Fig. 8c) shows only two peaks
attributed to the rutile 101 and 202 reflections, giving unambiguously the film growth orientation. The rocking curve
recorded around the 101 peak has ! = 1.38 . The ’ scan
recorded for the (002) plane of the 101-oriented rutile showed
only one peak (’ = 1.84 ) at 180 from the (0006) reflection
of the substrate (not shown), implying the following relations:
ð101Þrutile ==ð1102Þsapphire and ½010rutile ==½100sapphire
and ½101rutile ==½121sapphire :
ð11Þ
As seen in Fig. 11, the rectangular mesh of TiO2 (4.59 5.46 Å) fits reasonably the substrate surface mesh [4.76 (2
5.06 Å)] with = 3.6 and 7.9%, respectively. Moreover a very
good fit is observed at the atomic scale as both the metallic and
the oxygen networks closely coincide. Note that both the
substrate and the film structure are asymmetric in the interface
plane, explaining the fact that in this case no twinning occurs.
3.3.4. Film morphology. Fig. 12 displays SEM secondary
electron images for TiO2 films grown on M-, C- and
R-sapphire at 973 K. All samples presented a homogeneous
surface, composed by very small grains. The smallest grains
were obtained on M-sapphire, while on R-sapphire a more
jagged surface was observed. This difference can be understood in the context of growth dynamics. Indeed, it is established that the {110} surfaces of rutile have the lowest surface
energy and consequently tend to develop faster (Ramamoorthy et al., 1994). This will favor a small-grained fiber
texture for (100) rutile films, for which the {110} facets are
perpendicular to the substrate. Conversely, in the case of (101)
rutile, for which the fast-growing facets develop at 67.5 from
the substrate, a rough surface can be expected. This model
fully explains the detail of the microstructure recently
reported by Yeh et al. (2010).
Figure 10
Figure 11
Distribution as a function of deposition temperature of (001)- and (112)oriented anatase and (100)-oriented rutile crystallites for TiO2 films
grown on C-plane sapphire.
A sketch of the atomic coincidences at the interface between (101) rutile
and (1102) sapphire. For clarity, three unit cells of rutile and two of
sapphire are drawn and the atom sizes are not scaled.
J. Appl. Cryst. (2010). 43, 1502–1512
V. F. Silva et al.
Substrate-controlled allotropic phases
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4. Discussion and conclusions
Titanium oxide is among the most studied materials worldwide, because of its extremely promising applications, and
there are obviously a lot of studies related to TiO2 thin films
grown by various methods, including PLD. However, because
the existing literature data cover a broad range of different
methods of deposition, different conditions, different
substrates etc., we have carried out a systematic study,
choosing a single technique of film growth (pulsed laser
deposition) and a limited range of temperature, but a wide
variety of (single-crystal) oxide substrates, including LaAlO3,
two different orientations of SrTiO3 and MgO, and three
Figure 12
FE-SEM images of TiO2 thin films deposited at 973 K on (a) M- (b) Cand (c) R-sapphire substrates.
1510
V. F. Silva et al.
Substrate-controlled allotropic phases
Table 1
Summary of results obtained in the present work on the different
substrates.
All films are epitaxial. Unless stated otherwise, the deposition temperature is
873 K.
Phase
Substrate
Rutile
Anatase
STO(100)
LAO(100)
MgO(100)
773 K
873 K
973 K
STO(110)
MgO(110)
773 K
873 K
973 K
M-Sapphire
C-Sapphire, 773–973 K
R-sapphire
–
–
(001)
(001)
–
–
(110)
–
(100)
(100)
(100)
(102)
–
–
(110)
(001)
(100)
(101)
(110) + (102)
(102)
(102)
–
(001) + (112)
–
orientations of sapphire. Additionally, the effect of deposition
temperature was checked in selected examples, owing to the
existence of several allotropes of TiO2 (mainly anatase and
rutile).
Epitaxial anatase and rutile phases were selectively
obtained and our results are summarized in Table 1. The
crystalline orientation and quality strongly depend on the
substrate nature, as well as its orientation. (100)LAO and
(100)STO substrates provided (001) anatase films presenting
very good crystalline quality. For the same deposition
temperature of 873 K, (100)- and (110)MgO substrates gave
oriented but more misaligned microstructures, very probably
as a result of the high misfit, of (100) and (102) anatase,
respectively. In contrast on (110)STO, high-quality epitaxial
(102) anatase was observed. On R- and M-sapphire substrates,
an epitaxial growth of rutile was obtained with (101) and (001)
orientations, respectively. In contrast, on C-sapphire three
types of crystallites, all epitaxied, coexist: (001) anatase, (112)
anatase and (100) rutile. These results show that the film–
substrate interactions are able to promote either the rutile or/
and the anatase allotropes, quite independently of the growth
temperature. Nevertheless, increasing the temperature tends
to favor the rutile allotrope, as clearly shown, for instance, in
the examples of films grown on (100) and (110) MgO or
C-plane sapphire. This result is obviously in agreement with
the thermodynamics of the anatase–rutile equilibrium.
However, it is worth noting that this equilibrium can also be
affected by doping effects that change the temperature transition (Rao et al., 1959; Gennari & Pasquevich, 1998; Rodriguez-Talavera et al., 1997).
Table 2 summarizes some examples of literature data and
shows that our results are in general agreement with them: this
is the case on (100)- and (110)STO, (100)MgO, and (100)LAO,
as well as on M- and R-sapphire, although polycrystalline
samples were reported in some examples. The main differences concern, for instance, the (102) anatase growth on (110)
MgO; only a few reports on the growth of TiO2 on this
J. Appl. Cryst. (2010). 43, 1502–1512
research papers
Table 2
laser wavelength and a fluence lower than
ours by an order of magnitude.
Unless stated otherwise, films are epitaxial. A and R refer to anatase and rutile phases, respectively.
Another illustration of the effect of
Poly: polycrystalline without orientation; PLD: pulsed laser deposition; REBE: reactive electron
experimental conditions is given by the
beam evaporation; IBSD: ion beam sputter deposition; Sputt: direct current magnetron sputtering;
results of Yamamoto et al. (2001): using a
ALD: atomic layer deposition; MBE: molecular beam epitaxy.
KrF laser, they obtained, like us, (001) rutile
(a) (100)-oriented substrates.
on M-sapphire, while in their first report
(100)STO
(100)LAO (100)MgO
Method References
they obtained, in contrast, (101) rutile by the
use of a second harmonic Nd/YAG laser.
A(001)
A(001)
A(100)
PLD
Yamamoto et al. (2001)
A(001)
A(001)
–
REBE Lotnyk et al. (2007)
Also note that Kitazawa et al. (2006)
A(001) + weak
A(001)
–
PLD
Kennedy & Stampe (2003)
reported a mixture of (001) anatase + (100)
textured R(111)
rutile for PLD films grown on C-sapphire
–
A(001)
–
PLD
Kitazawa et al. (2006),
Sakama et al. (2006),
and found, for the same deposition
Park et al. (2002)
temperature, an increase of the rutile/
–
A(001)
–
Sputt
Singh et al. (2008)
anatase ratio when increasing the fluence.
–
–
A(100)
IBSD Morris Hotsenpiller et al. (1996)
–
–
A(100)
ALD
Schuisky et al. (2002),
Incidentally, at low pressure (10 Pa), they
Mitchell et al. (2005)
observed an unidentified peak, which could
PLD
Garapon et al. (1996)
–
–
Partially oriented
in fact be assigned to (112) anatase, as in this
A(001) + weak
R(200) and R(111)
report.
In summary, we succeeded in the epitaxial
growth of all these films, evidenced by an
(b) (110)-oriented substrates.
accurate X-ray diffraction study, including
(110)STO
(110)MgO
Method
References
rocking curves and ’ scans. We have shown
Poly mixed A+R
Poly mixed A+R
PLD
Yamamoto et al. (2001)
that we can control orientations as diverse as
A(012)
–
REBE
Lotnyk et al. (2007)
(001), (102), (100), (110) and (112) anatase
A(102)
–
PLD
Kennedy & Stampe (2003)
–
R(110)
IBSD
Morris Hotsenpiller et al. (1996)
and (110), (001), (100) and (101) rutile. In
some cases, where several phases and/or
orientations coexist, we evaluated their
(c) Sapphire substrates.
volumic fraction, a parameter that would be
M-Sapphire C- Sapphire
R-Sapphire Method References
of importance for further studies. MeanR(101)
R(100)
R(101)
PLD
Yamamoto et al. (2001)
while, a major target of this study was the
R(001)
R(100)
–
PLD
Yamamoto et al. (2002)
accurate determination of epitaxial relaR(001)
R(100)
R(101)
IBSD
Morris Hotsenpiller et al. (1996)
tionships between the film and the substrate,
–
R(100) + textured
–
PLD
Yamaki et al. (2002)
A(001)
the comprehension of what happens at the
–
A(001) + R(100)
–
PLD
Kitazawa et al. (2006),
interface, and the building of interfacial
Sbai et al. (2007)
models. The example of TiO2 growth on
–
R(100)
–
PLD
Xin et al. (2006)
–
R(100)
–
Sputt
Singh et al. (2008)
R-plane sapphire is of special interest in this
–
–
R(101)
MBE
Engel-Herbert et al. (2009)
context: remember that this substrate is
–
–
R(101)
ALD
Schuisky et al. (2002)
commonly chosen for the growth of (100)oriented perovskites (Rousseau et al., 2008)
or perovskite-related materials (Thivet et al., 1994). Highsubstrate are available, and our finding of (102) anatase is
quality (101) rutile films are grown on this substrate, even at a
unique, probably because it is impossible to assess this
temperature as low as 873 K. For comparison, at the same
orientation without the help of pole figures. Morris Hotsentemperature, (001) anatase is observed on the (100)STO
piller et al. (1996) reported a (110) rutile growth (also present
perovskite substrate, and even on (100)LAO (which is in fact
in our study for the highest temperature), but one can guess
the R plane of the latter material in its actual trigonal structhey missed the possible presence of (102) anatase for the
above-mentioned reason. Also, on C-sapphire, we observed
ture, but presents an arrangement close to the perovskite one).
very reproducibly the previously unreported epitaxial growth
The preferential growth of (101) rutile on R-sapphire has then
to be related to the structural identity of the two materials at
of (112) anatase crystallites, together with (001) anatase and
the interface (refer to Fig. 11) and is a very illustrative
(100) rutile. This means that, additionally to the primary
example of substrate-induced structure stabilization.
effect, namely the substrate structure-induced growth,
secondary effects related to deposition conditions can be
invoked: for instance by direct current magnetron sputtering,
Singh et al. (2008) observed only (100) rutile on C-sapphire at
a deposition temperature of 923 K. More unexpectedly,
The authors acknowledge the CAPES-COFECUB (project
Yamamoto et al. (2001) obtained this unique orientation by
No. 644/09) for financial support, and I. Peron and J. Le Lannic
PLD, but it is worth noting that they worked with a different
for obtaining SEM images at CMEBA, University of Rennes.
Some examples of literature data on TiO2 thin films grown on substrates.
J. Appl. Cryst. (2010). 43, 1502–1512
V. F. Silva et al.
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1511
research papers
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